#include <bits/stdc++.h>
using namespace std;

#define int int64_t
#define endl "\n"

using PII = pair<int, int>;
using TII = tuple<int, int, int>;
template<class T> using V = vector<T>;
using i64 = long long;
using i128 = __int128;
using u64 = unsigned long long;

#define FOR(i, a, b) for (int i = (int)(a); i <= (int)(b); i++)
#define FOR2(i, a, b, c) for (int i = (int)(a); i <= (int)(b); i += c)
#define FORD(i, b, a) for (int i = (int)(a); i >= (int)(b); i--)
#define FORD2(i, b, a, c) for (int i = (int)(a); i >= (int)(b); i -= c)
#define ALL(a) a.begin(), a.end()
#define RALL(a) a.rbegin(), a.rend()
#define lowbit(x) ((x) & (-x))
#define RE return;
#define Yes cout << "Yes" << endl;
#define YES cout << "YES" << endl;
#define No cout << "No" << endl;
#define NO cout << "NO" << endl;
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define sz(x) (int)(x).size()

void solve() {
    int n, k;
    string s;
    cin >> n >> k >> s;
    s = ' ' + s;

    V<int> pre(n + 1, 0);
    FOR(i, 1, n) {
        pre[i] = pre[i - 1] + (s[i] == '1');
    }

    V<int> c;
    FOR(i, 1, n) {
        if (s[i] == '0') {
            c.eb(i);
        }
    }

    string ans;
    bool f = false;

    for (auto x : c) {
        int t = pre[x];
        if (k >= t) {
            ans += '0';
            k -= t;
        } else {
            int p = t - k;
            while (p--) {
                ans += '1';
            }
            ans += '0';
            while (k--) {
                ans += '1';
            }
            FOR(i, x + 1, n) {
                ans += s[i];
            }
            f = 1;
            break;
        }
    }

    if (!f) {
        int t = pre[n];
        while (t--) ans += '1';
    }

    cout << ans << endl;
}

signed main() {
    int Task = 1;

    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin >> Task;
    while (Task--) {
        solve();
    }

    return 0;
}
